A. Technical Field
The present invention relates to switching regulators, and more particularly, to systems, devices, and methods of detecting and adjusting zero cross in switching regulators.
B. Background of the Invention
Switching regulator design for mobile device applications trends toward reducing component size and developing regulators that perform at higher and higher switching frequencies that allow designers to reduce the area that a circuit occupies on a board, especially by shrinking the size of inductors.
Switching regulator designers have been seeking solutions that allow to reduction in the size of external components as well as embed, for example, the LC filter in the same package as the control circuitry without compromising efficiency. Buck switching regulators, in particular, provide improved efficiency for relatively light load currents when operated in discontinuous conduction mode. Discontinuous conduction mode operation, however, typically requires the implementation of some type of current sensing circuitry to detect and ideally prevent a reverse current from flowing from the output of the filter through the inductor back into the synchronous rectifier and to ground. The task of designing an appropriate current sense circuit that accurately monitors the reverse inductor current becomes increasingly more difficult as switching frequencies continue to increase.
Existing methods to sense and minimize reverse current involve complex and, hence, costly circuitry. One conventional method uses a sense resistor in series with the output inductor. One method uses an RC network that mimics the time constant of the inductor and its parasitic resistance to detect when the current through the inductor reaches zero. Other methods use the on-resistance, RDS—ON, of the power FET within the switching regulator to measure the voltage across the power FET in order to derive an inductor current thereform.
While using RDS—ON as a sense resistor allows elimination of another resistive component that would further degrade efficiency, this approach suffers from propagation delays and requires a blanking time while waiting for the switching node voltage at the output inductor to settle after turning on the power FET of the synchronous rectifier before the voltage across the power FET can be sensed. Propagation delay makes this method of preventing reverse current flow through the inductor back into the regulator in response to detecting a zero crossing impractical at frequencies higher than about 6 MHz. What is needed are tools for switching regulator designers to overcome the above-described limitations.